1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copying machine and a facsimile, which utilizes an electrophotographic process or an electrostatic recording process.
2. Description of the Related Art
In a known electrophotographic image forming apparatus such as a copying machine and a printer, an electrostatic latent image (also simply called a latent image) is formed by exposing an electrophotographic photosensitive member (also simply called a photosensitive member), which has been charged, by using an optical writing unit, e.g., a laser. Then, a developer bearing member in a developing apparatus causes a developer to adhere to the latent image so that the latent image is visualized as a developer image. The developer adhering to the photosensitive member is transferred to a recording medium, e.g., a sheet of paper, whereby an output image is formed.
Methods of developing the latent image to the developer image are mainly divided into a one-component development method and a two-component development method. The one-component development method uses, as a developer (developing agent), a one-component developer that is substantially made of only resin toner particles (toner). On the other hand, the two-component development method uses, as the developer, a two-component developer that is primarily made of resin toner particles (toner) and magnetic carrier particles (carrier).
Further, the one-component development method is divided into a contact development method of bringing a toner layer formed on a developer bearing member into contact with an image bearing member in a development region, to thereby develop the latent image, and a noncontact development method of holding the developer bearing member and the image bearing member in a noncontact relation and causing toner particles to fly from the developer bearing member toward the image bearing member, to thereby develop the latent image.
FIG. 3 is an enlarged explanatory view illustrating movements of toner in the development region in which the image bearing member and the developer bearing member are positioned to face each other, when the noncontact development method is used. In FIG. 3, at an edge where there is a gap in potential of the latent image on the image bearing member, toner (t) positioned on the developer bearing member opposing to the image bearing member near the edge is attracted toward a high-density image portion. As illustrated in FIG. 6, therefore, the amount of toner adhering to the high-density image portion near the edge is increased beyond the estimated amount. Consequently, the image density is too increased, or an area where the toner (t) adheres to the high-density image portion is too widened (such a phenomenon is referred to as an “edge effect” hereinafter).
Also, because the toner (t) having adhered to a low-density image portion adjacent to the high-density image portion is attracted toward the high-density image portion, the toner (t) is less apt to adhere to the low-density image portion, thus generating a white spot (or streak) (such a phenomenon is referred to as a “white spot” hereinafter).
The edge effect and the white spot are more noticeably generated with the noncontact development method. The mechanism for generation of the edge effect and the white spot will be described in more detail with reference to FIGS. 3 and 6.
In an air layer through which the toner having departed from the developer bearing member flies and adheres to the image bearing member, lines of electric force are generated in a region positioned above and spanning from the low-density image portion to the high-density image portion (FIG. 3). The lines of electric force act perpendicularly to equi-potential lines illustrated in FIG. 3. Because the toner (t) on an area of the developer bearing member opposing to the low-density image portion flies along the lines of electric force, the amount of toner adhering to the high-density image portion near the edge is increased beyond the amount estimated from the potential of the latent image (FIG. 6). On the other hand, the amount of toner adhering to the low-density image portion near the edge is reduced beyond the amount estimated from the potential of the latent image.
More specifically, when an AC continuous wave is applied to the developer bearing member, the direction of an electric field generated by an AC voltage is changed per half cycle and the direction of movement of the toner (t) is also changed per half cycle. Therefore, the toner flying speed is attenuated near a point where the direction of movement of the toner (t) is changed, so that the toner (t) is more apt to move along the lines of electric force illustrated in FIG. 3. As a result, the amount of toner adhering to the high-density image portion near the edge is increased and the edge effect is generated. With the generation of the edge effect, the amount of toner adhering to the low-density image portion near the edge is reduced and the white spot is generated.
Another cause of the edge effect and the white spot resides in that, even with the toner (t) flying toward the low-density image portion of the image bearing member, the toner (t) repeats bounding on the image bearing member and is finally settled onto the high-density image portion.
The edge effect and the white spot are not generated in ideal development (i.e., development with charging efficiency of 100%) in which the potential difference between the high-density image portion and the low-density image portion is completely canceled with adhesion of the toner in a developing step. The reason is that, even when the edge effect and the white spot are generated through the above-described process during the developing step, the lines of electric force acting in the direction to generate the edge effect and the white spot are weakened with the progress of the development and a development result is finally obtained as per the potential of the latent image. In an ordinary system, however, the charging efficiency in the developing step is not sufficient and the developing step is ended while the potential difference between the high-density image portion and the low-density image portion remains to some extent. Hence the edge effect and the white spot also remain at the time of end of the development.
Further, in the noncontact development method, because a developer layer on the developer bearing member and a surface of the image bearing member are maintained in a noncontact state, a distance (gap) between the developer bearing member and the image bearing member is required to be set larger than that in the contact development method. With the setting of the larger distance, the lines of electric force are curved to a larger extent, thus making the edge effect and the white spot more noticeable.
The edge effect and the white spot are also generated in the case of contact development, though not so noticeable as in the case of noncontact development.
Aiming to avoid the edge effect and the white spot, there is a known technique (U.S. Pat. No. 5,311,262) for specifying conditions of magnetic poles in a magnetic developing apparatus. However, the known technique cannot be applied to a nonmagnetic developing apparatus, and it requires the magnetic poles to be designed in complicated arrangement.
Other known proposals include a method of inspecting the degree of development at the edge by using a patch and making correction with image processing (Japanese Patent Laid-Open No. 6-60181) and a method of correcting the directions of the lines of electric force by forming irregularities on a developer bearing member (Japanese Patent Laid-Open No. 10-171238). However, those known methods require complicated arrangements and increase the cost when practiced.
As another solution, there is known a method of using an AC waveform having blank portions as a bias voltage applied to the developer bearing member. Such an AC waveform is called a blank pulse waveform obtained by modifying a bias waveform of an AC voltage superimposed with a DC waveform such that an AC component is intermittently paused to provide blank portions and only a DC component is applied in the blank portions.
An example of the known development method using the blank pulse waveform as the bias voltage for the development is described in U.S. Pat. No. 5,669,050.
That known development method is advantageous in avoiding the edge effect and the white spot. In that known development method, however, the AC voltage acting to trigger release of the toner from the surface of the developer bearing member is not applied in the blank portions in which the application of the AC voltage is paused. This causes the problem that, in the blank portions, the amount of released toner is reduced and development performance (toner adhesion) is entirely deteriorated in comparison with continuous waveform portions other than the blank portions.
In view of the above-mentioned problem, a method of making the intermittently paused AC component asymmetrical is proposed which is advantageous in not only avoiding the edge effect and the white spot, but also preventing the deterioration of the development performance (see U.S. Pat. No. 6,285,841). Although the proposed method can prevent the deterioration of the development performance, it has the problem that stability in the development process is impaired due to the use of intermittent pulses and sharpness of an image is lost.